1. Field of the Invention
The present invention relates to a method for producing a graded-refractive-index optical fiber (hereinafter sometimes referred to as GI type optical fiber for short) having high transparency and high heat resistance simultaneously, from a base material (preform), which was difficult to attain with conventional optical plastics.
The GI type optical fiber obtained by the present invention is an amorphous resin and thus is free from light scattering and very transparent to light at wavelengths within a wide range from ultraviolet light to near infrared light. Therefore, it is useful for optical systems for light of various wavelengths. In particular, the GI type optical fiber obtained by the present invention provides a light transmission medium with small losses at wavelengths of 1,300 nm and 1,550 nm, at which a trunk vitreous silica fiber is used in the field of optical communication.
2. Description of the Background
It has been known by JP-A-8-334633 to produce the GI type optical fiber by using an amorphous fluoropolymer (a) which has no C--H bond and a substance (b) which differs in refractive index by at least 0.001 in such a manner that a cylindrical molded body made of a low-refractive-index material is used as a mold, and made to contain a layer-forming material having a high refractive index on its inner surface, and it is subjected to rotational molding so that the layer-forming material having a high refractive index thermally diffuse during the rotational molding to form a cylindrical base material having an inner and outer at least two layer structure; and the obtained cylindrical base material is then formed into a fiber.
In the above method for production of the GI type optical fiber, by making the base material cylindrical, there will be no voids, bubbles or deformation resulting from cooling shrinkage of the resin, and no light scattering body will form, and thus, the transmission property improves. However, there was a defect that the hollow part is likely to remain in the fiber when the cylindrical base material is formed into a fiber.
Further, there was a defect that it is difficult to optionally change the refractive index distribution form, and in many cases, the distribution form is determined uniformly, and it is difficult to obtain the refractive index distribution form suitable for the required property of e.g. a light source, a light receiver or a connector.